(1) Field of the Invention
The present invention relates to a process for the preparation of a plastic type multifilament optical fiber comprising, as an island component, 1,000 to 10,000 filaments having a diameter smaller than 500.mu., especially 50 to 300.mu., which are embedded and arranged in a plastic material as a sea component.
(2)Description of the Related Art
A multifilament optical fiber comprising quartz type optical filaments having a diameter smaller than 200.mu., which are arranged at a high degree of orientation and having ends thereof bonded to one another by an adhesive, is capable of transmitting an image by light and is widely used as an image scope in the field of medical instruments and as an optical fiber sensor in other industrial fields.
This quartz type multifilament optical fiber is formed by gathering quartz type optical filaments independently prepared. This quartz type optical fiber is disadvantageous in that if there is a slight arrangement error in optical filaments, the optical fiber cannot be used as an image transmitting member at all. Accordingly, a very high precision is required for the preparation process and hence, the manufacturing cost is extremely increased. Furthermore, the quartz type optical fiber is more readily broken than a plastic type optical fiber, and even if one filament is damaged during the preparation of the multifilament quartz type optical fiber, the optical fiber cannot be used as an image transmitting member at all. Accordingly, strict control is required for maintenance of the preparation process. Moreover, quartz type optical filaments are readily broken and this means that the quartz type multifilament optical fiber is often rigid. Therefore, when the optical fiber is used as an image scope, a large bending angle cannot be adopted and the visual field that can be inspected becomes narrow.
On the other hand, a plastic type optical fiber is advantageous in that it is soft and easy to handle, and various multifilament optical fibers comprising plastic optical filaments have been developed. For example, U.S. Pat. No. 3,556,635 discloses a process in which a plastic type multifilament optical fiber is prepared by using a composite spinning nozzle as shown in FIG. 1. This spinning nozzle comprises a core-forming orifice plate 11, a sheath-forming orifice plate 12 and a sea-forming orifice plate 13, which are combined so that spaces 14 and 15 are formed, and pipes 16 and 17 arranged vertically in the respective spaces to introduce the respective components. The multifilament optical fiber prepared according to this process comprises about 100 filaments having a cross-sectional structure shown in FIG. 3 of this patent and described in column 5, lines 23 through 34 and also having a diameter of 100.mu., and the matrix occupies about 45% of the total volume. The plastic type multifilament optical fiber can tentatively transmit an image, but since the number of filaments per bundle is as small as about 100 and the matrix content is as high as 45%, the resolving power is smaller than that of the quartz type multifilament optical fiber. Therefore, development of a plastic type multifilament optical fiber having a high optical filament pack density is desired. In addition, the above-mentioned known process is defective in that sheath and sea polymers are likely to stay in dead spaces 18 and 19 of the spinneret. Moreover, the pipes 16 and 17 are arranged for introduction of the respective components and the cross-section of the plastic type multifilament optical fiber prepared by using this spinneret nozzle tends to have a square, hexagonal or octagonal shape. This is another cause of the insufficient image transmitting property of this multifilament optical fiber.
Japanese Unexamined Patent Publication (Kokai) No. 54-116417 teaches that if an improved spinneret 20 as shown in FIG. 2 is used, dead spaces for polymers are not formed at all and an improved plastic type multifilament optical fiber can be obtained, and Japanese Unexamined Patent Publication No. 56-39505 shows a plastic type multifilament optical fiber prepared by using this spinning apparatus. However, the number of optical filaments in the plastic type multifilament type optical fiber prepared according to this process is smaller than 200 and the matrix content is as high as 20 to 30%. Moreover, deviations of the cross-sectional shape and fineness are great among the optical filaments as the island component, and therefore, a further improvement is required so as to utilize this plastic type multifilament optical fiber as an image transmitting member.